Apparatus and Methods for Programmable Logic Devices with Improved Performance Characteristics

ABSTRACT

Apparatus and methods are disclosed for improving the performance of a programmable logic device (PLD). A PLD includes a memory cell configured to provide a pair of voltages to a gate of a pass transistor and a body of the pass transistor, respectively.

TECHNICAL FIELD

The inventive concepts relate generally to circuitry and associatedmethods for improving the performance of electronic circuitry. Moreparticularly, the invention concerns apparatus and associated methodsfor improving performance characteristics of programmable logic devices(PLDs) or portions of PLDs, such as multiplexers (MUXs) included inPLDs.

BACKGROUND

Modern PLDs include a relatively large number of pass transistors. Thepass transistors may reside in a variety of PLD circuits, such as MUXsused in various PLD blocks. The pass transistors typically transmit asignal from one of their terminals (say, the source) to another terminal(say, the drain) in response to a control signal (e.g., a voltageapplied to the gate terminal). Because of technological advances indevice fabrication techniques, device features have shrunk in size(sometimes referred to as “technology scaling”). As a relatedphenomenon, supply voltages have decreased (also known as “voltagescaling”).

Threshold voltages of the transistors, however, have failed to scale ina proportionate manner. In other words, threshold voltages havedecreased, although at a slower rate or pace. Consequently, theperformance of the pass transistors has tended to suffer, especially inthe second stages of MUXs, as the threshold voltages preclude ahigh-enough drive-current capability, given the supply voltage levelstypically available. A need therefore exists for improving theperformance of PLDs or portions of PLDs, such as pass transistors usedin MUXs or other circuitry.

SUMMARY

The disclosed novel concepts relate to apparatus and methods forimproving the performance characteristics of PLDs or portions of PLDs,such as MUXs that use pass transistors, etc. In one embodiment, a PLDincludes a memory cell that is configured to provide one voltage to agate of a pass transistor and another voltage to a body of the passtransistor. In another embodiment, a circuit arrangement includes acomposite configuration memory cell, and a pass transistor coupled tothe composite configuration memory cell. The composite configurationmemory cell supplies a voltage to a gate of the pass transistor. Thecomposite configuration memory cell supplies another voltage to a bodyof the pass transistor.

Yet another embodiment relates to a method of coupling resources withina PLD via a pass transistor, which depends on the state of aconfiguration memory cell. The method includes supplying one set ofvoltages to the gate and the body of the pass transistor when theconfiguration memory cell is in one state, and supplying another set ofvoltages to the gate and the body of the pass transistor when theconfiguration memory cell is in another state.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate only exemplary embodiments of theinvention and therefore should not be considered or construed aslimiting its scope. Persons of ordinary skill in the art who have thebenefit of the description of the invention appreciate that thedisclosed inventive concepts lend themselves to other equally effectiveembodiments. In the drawings, the same numeral designators used in morethan one drawing denote the same, similar, or equivalent functionality,components, or blocks.

FIG. 1 shows a general block diagram of a PLD according to anillustrative embodiment of the invention.

FIG. 2 illustrates a conventional MUX used in traditional PLDs.

FIG. 3 depicts a simplified block diagram of a composite configurationmemory (CRAM) for supplying voltages to a corresponding pass transistor.

FIG. 4 shows a circuit arrangement for a composite CRAM and acorresponding pass transistor according to an exemplary embodiment ofthe invention.

FIG. 5 depicts a pass transistor arrangement according to anillustrative embodiment of the invention used to couple two PLDresources to one another.

FIG. 6 illustrates selective application of voltages generated bycomposite CRAM cells according to the invention to respective passtransistors in a PLD.

DETAILED DESCRIPTION

The inventive concepts contemplate apparatus and methods for improvingperformance characteristics of PLDs. More specifically, the disclosednovel concepts provide techniques for overcoming performance limitationsof pass transistors in PLDs, for example, pass transistors in MUXs(especially second-stage pass transistors, and especially during a0-to-1 transition). The inventive concepts improve the performance ofpass transistors (and, hence, the performance of MUXs or other circuitsthat include the pass transistors) by providing separate gate and bodyvoltages to the pass transistors, as described below in detail.

One may apply the inventive concepts to a variety of programmableintegrated circuits, such as PLDs. FIG. 1 shows a general block diagramof a PLD 103 according to an illustrative embodiment of the invention.PLD 103 includes configuration circuitry 130, configuration memory(CRAM) 406, control circuitry 136, programmable logic 106, programmableinterconnect 109, and I/O circuitry 112. CRAM 406 provides a pluralityof voltages to all or a selected number of pass transistors in PLD 103,as described below in detail.

In addition, PLD 103 may include test/debug circuitry 115, one or moreprocessors 118, one or more communication circuitry 121, one or morememories 124, one or more controllers 127, as desired. The choice andnumber of the blocks depends on a number of factors, such as design andperformance specifications, as persons of ordinary skill in the art whohave the benefit of the description of the invention understand.

Programmable logic 106 includes blocks of configurable or programmablelogic circuitry, such as look-up tables (LUTs), product-term logic,multiplexers (MUXs), logic gates, registers, memory, and the like.Programmable interconnect 109 couples to programmable logic 106 and toother blocks and circuitry within PLD 103, as desired. As describedbelow in detail, programmable interconnect 109 provides configurableinterconnects (coupling mechanisms) between various blocks withinprogrammable logic 106 and other circuitry within or outside PLD 103.

Control circuitry 136 controls various operations within PLD 103. Underthe supervision of control circuitry 136, PLD configuration circuitry130 uses configuration data (which it obtains from an external source,such as a storage device, a host, etc.) to program or configure thefunctionality of PLD 103. Configuration data are typically used to storeinformation in CRAM 406. The contents of CRAM 406 determine thefunctionality of various blocks of PLD 103, such as programmable logic106 and programmable interconnect 109.

I/O circuitry 112 may constitute a wide variety of I/O devices orcircuits, as persons of ordinary skill in the art who have the benefitof the description of the invention understand. I/O circuitry 112 maycouple to various parts of PLD 103, for example, programmable logic 106and programmable interconnect 109. I/O circuitry 112 provides amechanism and circuitry for various blocks within PLD 103 to communicatewith external circuitry or devices.

Test/debug circuitry 115 facilitates the testing and troubleshooting ofvarious blocks and circuits within PLD 103. Test/debug circuitry 115 mayinclude a variety of blocks or circuits known to persons of ordinaryskill in the art who have the benefit of the description of theinvention. For example, test/debug circuitry 115 may include circuitsfor performing tests after PLD 103 powers up or resets, as desired.Test/debug circuitry 115 may also include coding and parity circuits, asdesired.

PLD 103 may include one or more processors 118. Processor 118 may coupleto other blocks and circuits within PLD 103. Processor 118 may receivedata and information from circuits within or external to PLD 103 andprocess the information in a wide variety of ways, as persons skilled inthe art with the benefit of the description of the invention appreciate.One or more of processor(s) 118 may constitute a digital signalprocessor (DSP). DSPs allow performing a wide variety of signalprocessing tasks, such as compression, decompression, audio processing,video processing, filtering, and the like, as desired. As persons ofordinary skill in the art who have the benefit of the description of theinvention understand, rather than using a dedicated DSP, one may use thelogic resources of PLD 103 to implement DSP functionality, as desired.

PLD 103 may also include one or more communication circuits 121.Communication circuit(s) 121 may facilitate data and informationexchange between various circuits within PLD 103 and circuits externalto PLD 103, as persons of ordinary skill in the art who have the benefitof the description of the invention understand.

PLD 103 may further include one or more memories 124 and one or morecontroller(s) 127. Memory 124 allows the storage of various data andinformation (such as user-data, intermediate results, calculationresults, etc.) within PLD 103. Memory 124 may have a granular or blockform, as desired. Controller 127 allows interfacing to, and controllingthe operation and various functions of circuitry outside the PLD. Forexample, controller 127 may constitute a memory controller thatinterfaces to and controls an external synchronous dynamic random accessmemory (SDRAM), as desired.

Note that FIG. 1 shows a simplified block diagram of PLD 103. Thus, PLD103 may include other blocks and circuitry, as persons of ordinary skillin the art understand. Examples of such circuitry include clockgeneration and distribution circuits, redundancy circuits, and the like.Furthermore, PLD 103 may include, analog circuitry, other digitalcircuitry, and/or mixed-mode circuitry, as desired.

Currently, typical PLDs use metal oxide semiconductor (MOS) transistors(usually as part of a complementary MOS, or CMOS, arrangement) as passtransistors. A typical MOS transistor has source, drain, gate, and bodyterminals, as persons of ordinary skill in the art understand. The draincurrent of the MOS transistor in the saturation region of operationdepends on the threshold voltage and the gate-to-source voltage of thetransistor:i _(D) =K(v _(GS) −V _(T))²,where

i_(D)=the total drain current (i.e., including AC and DC components),

K=a constant,

V_(GS)=the total gate-to-source voltage (i.e., including AC and DCcomponents), and

V_(T)=the threshold voltage.

The threshold voltage, V_(T), depends on a number of factors, such asthe voltage between the source and body terminals of the transistor. Thefollowing equation provides the threshold voltage as a function of thebody-to-source voltage:$V_{T} = {V_{T{(0)}} + {\gamma\left\{ {\sqrt{{2\quad\phi_{F}} - v_{BS}} - \sqrt{2\quad\phi_{F}}} \right\}}}$where

V_(T(o))=the threshold voltage with the source-to-body voltage (orbody-to-source voltage) set to zero,

γ=the body factor, a constant that depends on the doping levels of thebody,

φ_(F)=a constant, and

v_(BS)=the total body-to-source voltage (i.e., including AC and DCcomponents).

Note that when the body-to-source voltage, v_(BS) equals zero, thethreshold voltage, V_(T), equals V_(T(o)). By adjusting thebody-to-source voltage, one may increase or decrease the thresholdvoltage of the transistor.

As the above equations show, the drain current and, hence, thecurrent-drive capability of the transistor for a given gate-to-sourcevoltage, depends on the threshold voltage. In other words, the higherthe threshold voltage, the less the current-drive capability of thetransistor, and vice-versa. Thus, to increase the transistor'scurrent-drive capability, one would normally desire to decrease itsthreshold voltage.

Decreasing the threshold voltage, however, increases the off-stateleakage current, I_(off). An elevated level of the off-state leakagecurrent has the undesirable effect of increased power consumption ordissipation of the transistor and, hence, of the PLD overall. As aresult, a trade-off exists between the value of the threshold voltage,as set by the physical characteristics of the transistor and by thebody-to-source voltage (and therefore the current-drive capability ofthe transistor) and its off-state leakage current. In conventionalapproaches, one would have to choose one factor versus the other, ortrade off one performance characteristic for the other.

FIG. 2 shows a conventional MUX, with the pass transistors biased in atraditional manner. More specifically, the first stage of the MUXincludes transistors 303A-303C, and the second stage of the MUX includestransistor 306. CRAM cells 133A-133D provide the gate voltages of thetransistors in the first and second stages of the MUX, respectively.Bias generator 309 provides a body bias to the transistors in both thefirst and second stages of the MUX.

As noted above, a trade-off exists between the current-drive capability(and speed of operation) of transistors 303A-303C and 306, and theiroff-state current. In the conventional circuit arrangement in FIG. 2,bias generator 309 provides a bias voltage to the bodies of thetransistors separately from the gate voltages supplied by CRAM cells133A-133D. For the reasons described above, the performance of the passtransistors tends to suffer, especially pass transistor 306 in thesecond stage of the MUX.

The inventive concepts provide a way of decreasing the gate voltageduring the on-state of the pass transistor, and decreasing the leakagecurrent during its off-state. More specifically, pass transistorsaccording to exemplary embodiments of the invention receive both theirgate voltages and body voltages from modified or composite CRAM cells.

FIG. 3 shows a simplified block diagram according to the invention of acomposite configuration memory (CRAM) for supplying voltages to acorresponding pass transistor. More specifically, composite CRAM 406provides voltage V_(c1) to the body of transistor 303/306. CompositeCRAM 406 further supplies a separate voltage V_(c2) to the gate oftransistor 303/306.

Composite CRAM 406 has two states. In the first state, voltage V_(c1)has a logic high value. That level of voltage V_(c1) turns ON transistor303/306. Voltage VC₂ has a value higher than a logic low value (e.g.,0.4 volts higher than logic low). That value of voltage V_(c2) biasesthe body of transistor 303/306 so as to reduce or minimize its thresholdvoltage.

More specifically, an elevated body bias increases the body-to-sourcevoltage which, according to the equation presented above, reduces thethreshold voltage of transistor 303/306. For a given gate-to-sourcevoltage (the gate drive voltage), the reduced threshold voltage resultsin an increased drain current, i.e., increased current-drive capability.

Simultaneously, the values of voltages V_(c1) and V_(c2) increase thesaturation drain current (I_(dsat)). Put another way, for a givengate-to-source voltage, the decrease in the threshold voltage causes thedrain saturation current to increase, thus aiding the current capabilityof transistor 303/306.

In the second state, both V_(c1) and V_(c2) have logic low values. Notethat, theoretically, one may apply a negative V_(c2), as desired. As aresult, the threshold voltage of transistor 303/306 increases. Given thezero or negative body bias and the reduced gate-to-source voltage, theoff-state leakage current (I_(off)) decreases or minimizes. Thereduction in the off-state leakage current helps to improve theperformance of the PLD by decreasing its overall power consumption.

FIG. 4 shows a circuit arrangement for a composite CRAM and acorresponding pass transistor according to an exemplary embodiment ofthe invention. The circuit arrangement includes transistors 509 and 512,and inverters 503 and 506 in a back-to-back configuration. An output ofinverter 503 drives the gate of transistor 512, whereas an output ofinverter 506 drives the gate of transistor 509 and also provides voltageV_(C2) to the gate of pass transistor 303/306.

Depending on the output voltages of inverters 503 and 506, transistors509 and 512 supply either voltage V₁ or voltage V₂ to the body of passtransistor 303/306. More specifically, when the outputs of inverter 503and 506 have logic low and high values, respectively, transistor 509 isON, and transistor 512 is OFF. As a result, transistor 509 couplesvoltage V₁ (i.e., as V_(c2)) to the body of transistor 303/306.

Conversely, when the outputs of inverters 503 and 506 have logic highand low values, respectively, transistor 509 is OFF, and transistor 512is ON. As a result, transistor 512 couples voltage V₂ (i.e., as V_(c2))to the body of transistor 303/306.

By selecting appropriate values of voltages V₁ and V₂, one may providethe advantages of low off-state leakage current and highcurrent-drive-capability for transistor 303/306, as described above. Asexample, voltages X may have the values specified in Table 1: TABLE 1Stage of Transistor Gate Voltage of Body Voltage of 303/306 Transistor303/306 Transistor 303/306 ON 1.0 V 0.4 V OFF 0.0 V 0.0 V

Note that, as persons of ordinary skill in the art who have the benefitof the description of the invention understand, voltages V₁ and V₂ mayhave a wide variety of other values, as desired. The choice of voltagevalues depends on a number of factors, such as the characteristics oftransistor 303/306, the desired design and performance specifications,and the like, as persons of ordinary skill in the art who have thebenefit of the description of the invention appreciate.

One may use the circuit arrangements in FIG. 3 or FIG. 4 to coupletogether various PLD resources, such as the blocks shown in FIG. 1 orother circuitry within the PLD, as desired. FIG. 5 depicts a passtransistor arrangement according to an illustrative embodiment of theinvention used to couple two PLD resources to one another.

More specifically, pass transistor 503 couples PLD resource 506 to PLDresource 509. Composite CRAM cell 406 provides two voltages totransistor 503, as described above. Pass transistor 503 allows PLDresource 506 and PLD resource 509 to communicate with each other withthe advantages described above, and without the shortcoming of theconventional approach.

Note that, rather than a single pass transistor, one may use a varietyof more complex arrangements, as desired, and as persons of ordinaryskill in the art who have the benefit of the description of theinvention understand. As one example, one may use an appropriate numberof composite CRAM cells to provide corresponding voltages to a number ofpass transistors arranged as a MUX. The MUX may provide part or all ofthe functionality of programmable logic 106, programmable interconnect109, or other desired functionality in PLD 103, as desired.

Note that one may use the composite CRAM cells (e.g., as shown in thecircuit arrangement of FIG. 3) selectively. In other words, for a givenblock or arbitrary circuit within PLD 103, one may use the conventionalapproach for some pass transistors and yet use composite CRAM cells forother pass transistors (or MUXs or other desired structures).

FIG. 6 illustrates selective application of voltages generated bycomposite CRAM cells according to the invention to respective passtransistors in a PLD. More specifically, FIG. 6 shows a three-input MUX,with inputs a, b, and c, and output z. Each input of the MUX couples toa corresponding pass transistor. Thus, input a couples to passtransistor 303A, input b couples to pass transistor 303B, and input ccouples to pass transistor 303C.

Composite CRAM cells 406A and 406B drive the gates and bodies oftransistors 303A and 303B, respectively. Conventional CRAM cell 133Cdrives the gate of transistor 303C. Bias generator 309 provides a biasto the body of transistor 303C. Thus, composite CRAM cells 406A-406Bprovide respective transistors 303A-303B with the benefits describedabove. Conventional CRAM 133C, however, drives transistor 303C. As aresult, transistor 303C has lower performance characteristics than dotransistors 303A-303B.

Note that composite CRAM cell 406C couples to transistor 306, and drivesits gate and body. That arrangement provides a higher-performance pathfrom inputs a and b to output z, while providing a lower-performancepath from input c to output z.

One may implement the inventive concepts using a variety ofsemiconductor structures, as desired, and as persons of ordinary skillin the art who have the benefit of the description of the inventionunderstand. For example, one may implement the novel circuitarrangements described above and shown in the drawings in bulk metaloxide semiconductor (MOS), complementary MOS (CMOS), orsilicon-on-insulator (SOI), as desired.

Furthermore, one may apply the inventive concepts to a variety of passtransistors, as desired, and as persons of ordinary skill in the art whohave the benefit of the description of the invention understand. Forexample, the pass transistors may constitute n-type MOS (NMOS)transistors, p-type MOS (PMOS) transistors (with modifications that fallwithin the knowledge of persons of ordinary skill in the art who havethe benefit of the description of the invention), or CMOS circuitarrangements, as desired.

Note that one may generally apply the inventive concepts effectively tovarious integrated circuits (ICs) (or even discrete logic), such ascustom ICs, and programmable ICs that include programmable orconfigurable logic circuitry, known by various names in the art, asdesired, and as persons skilled in the art with the benefit of thedescription of the invention understand. Such circuitry includes, forexample, devices known as complex programmable logic device (CPLD),programmable gate array (PGA), structured application specific ICs(structured ASICs), and field programmable gate array (FPGA).

Furthermore, one may use the inventive concepts in a variety ofelectronic circuitry and systems, such as SOCs and SOPCs, as desired.The details of implementation fall within the knowledge of persons ofordinary skill in the art who have the benefit of the description of theinvention.

Referring to the figures, persons of ordinary skill in the art will notethat the various blocks shown may depict mainly the conceptual functionsand signal flow. The actual circuit implementation may or may notcontain separately identifiable hardware for the various functionalblocks and may or may not use the particular circuitry shown. Forexample, one may combine the functionality of various blocks into onecircuit block, as desired. Furthermore, one may realize thefunctionality of a single block in several circuit blocks, as desired.The choice of circuit implementation depends on various factors, such asparticular design and performance specifications for a givenimplementation, as persons of ordinary skill in the art who have thebenefit of the description of the invention understand. Othermodifications and alternative embodiments of the invention in additionto those described here will be apparent to persons of ordinary skill inthe art who have the benefit of the description of the invention.Accordingly, this description teaches those skilled in the art themanner of carrying out the invention and are to be construed asillustrative only.

The forms of the invention shown and described should be taken as thepresently preferred or illustrative embodiments. Persons skilled in theart may make various changes in the shape, size and arrangement of partswithout departing from the scope of the invention described in thisdocument. For example, persons skilled in the art may substituteequivalent elements for the elements illustrated and described here.Moreover, persons skilled in the art who have the benefit of thisdescription of the invention may use certain features of the inventionindependently of the use of other features, without departing from thescope of the invention.

1. A programmable logic device (PLD), comprising a memory cell configured to provide a first voltage to a gate of a pass transistor and a second voltage to a body of the pass transistor.
 2. The programmable logic device (PLD) according to claim 1, wherein the pass transistor has improved current-drive capability.
 3. The programmable logic device (PLD) according to claim 1, wherein the pass transistor has improved off-state leakage current.
 4. The programmable logic device (PLD) according to claim 1, wherein the pass transistor resides in a multiplexer (MUX).
 5. The programmable logic device (PLD) according to claim 1, wherein the pass transistor resides in programmable logic circuitry within the PLD.
 6. The programmable logic device (PLD) according to claim 1, wherein the pass transistor resides in programmable interconnect circuitry within the PLD.
 7. The programmable logic device (PLD) according to claim 1, wherein the pass transistor couples a first resource within the PLD to a second resource within the PLD. 8-20. (canceled) 